1. Technical Field
The present disclosure relates to liquid crystal display, and more particularly to an LCD device and driving method thereof.
2. Description of Related Art
In a commonly used thin film transistor liquid crystal display (TFTLCD), content is displayed by rotating liquid crystal (LC) molecules inside the TFTLCD to specific attitudes to control a transparency (brightness) with adjusting bias voltages loaded to two sides of the TFTLCD. The LC molecules are permanently damaged and no longer rotate smoothly if electric fields generated by the bias voltages remain in the same direction for a long time. Hence, to prevent permanent damage, different driving methods with alternative directions of the bias voltages are provided, such as frame inversion, column inversion, line/row inversion and, dot inversion. Row inversion includes 1-line and 2-line row driving methods.
FIG. 11 shows a driving motion of the 1-line row inversion driving method. All pixels in a specific row in each frame have the same bias direction, with two adjacent rows having inverse (opposite) bias direction. That is, the rows during each frame are driven in alternate bias directions. During a subsequent frame period, all pixels of the specific row have an inverted bias direction. Therefore, the bias direction of each pixel of the TFTLCD is alternately driven frame by frame.
With reference to FIGS. 12 and 13, a 1-line row flicker pattern and a driving motion analysis corresponding to the flicker pattern are shown. In FIG. 12, empty boxes represent pixels displaying identical brightness, and shaded blocks present pixels displaying an unilluminated state. In FIG. 13, pixels in the illuminated state are marked by circles, and pixels in the unilluminated state are shown without circles. The TFTLCD successively displays images corresponding to the frames “n”, “n+1”, “n+2” . . . and so on, and the bias direction of each specific row is driven alternatingly. As shown in FIG. 13, a first row is continuously in the unilluminated state but is successively driven by bias voltages having negative (“−”), positive (“+”) and negative (“−”) bias directions respectively during frames n, n+1 and n+2. Hence, LC molecules in the rows are preserved with good characteristics.
Typically, a sequential square wave is input to a common electrode of the TFTLCD as a Vcom signal, referred to common voltages hereinafter. Periods of the Vcom signal during each frame are the same. Driving voltages applied to electrodes of an array side of the pixels in the TFTLCD correspond to the Vcom signal, whereby a bias voltage and a direction of the bias voltage to each pixel is determined. When voltage of the Vcom signal is lower than the driving voltage of a specific pixel, the bias voltage of the specific pixel is defined as being in the positive (“+”) bias direction. Otherwise, the bias voltage is defined as being in the negative (“−”) bias direction. However in practical use, the common voltages are often shifted and form a non-stable waveform frame by frame. Therefore, brightness of each pixel in one row is slightly changed with transformation of the frames when the TFTLCD displays a static picture as shown in FIG. 13. Hence, since the brightness change in the pixels during frame transformation is visible, flicker occurs.
Referring to FIG. 14, the double-line row inversion diving method is disclosed to solve the flicker problem. The bias voltages of adjacent rows during each frame have the same bias direction. For instance, the bias voltages of the 2n−1 row (where n is an integer) of the TFTLCD has the same bias direction as that of the 2n row. The bias voltages of the 2n+1 and 2n+2 rows have the same bias direction, but have inverse bias direction to the bias direction of the 2n−1 and 2n rows. Also, the bias voltage of each row has different bias direction frame by frame.
FIG. 15 shows a driving motion using the double-line row inversion driving method to solve the flicker problem of FIG. 12. Pixels with circles are defined in the illuminated state. In this example, half pixels in the illuminated state are driven by the bias voltages having positive bias direction, and the other of half pixels displaying the illuminated state are driven by the bias voltages having negative bias direction. The flicker problem is then solved since the brightness of pixels displaying the illuminated state compensate to each other in each frame, such that, the brightness changes (flicker) during frame transformation are no longer discernible.
Unfortunately, the double-line row inversion driving method is not flicker free when displaying sequential frames having two illuminated and two unilluminated rows. FIGS. 16 and 17 respectively show a flicker pattern of 2-line rows and a driving motion analysis corresponding to the same flicker pattern. Empty blocks in FIG. 16 represent pixels displaying illuminated states with identical brightness, and shaded blocks in FIG. 16 represent pixels displaying unilluminated states with the same brightness. In FIG. 17, pixels with illuminated states are marked by circles and pixels in unilluminated states are shown unmarked. Pixels having the same state (that is bright or unilluminated state) are driven by bias voltages having the same bias direction during each frame. In FIG. 17, all illuminated state pixels (circle marked) during frame “n” to “n+2” are alternately driven by bias voltages having positive, negative and positive bias directions respectively, such that the illuminated state pixels during frame transformation again suffer from irregular brightness and flicker.
Neither the 1-line row inversion nor the double line row inversion driving method is able to completely eliminate flicker entirely.
What is needed therefore, is a driving method and LCD that can overcome the limitations described.